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CHAPTER 202

INDIANA DEPARTMENT OF TRANSPORTATION 2013 DESIGN MANUAL CHAPTER 202 Hydrology Design Memorandum Revision Date Sections Affected 14-02 Feb. 2014 , Figure 202-3B, Figure 202-3C Page 2 2013 Indiana Design Manual, Ch. 202 TABLE OF CONTENTS LIST OF FIGURES .. 4 INTRODUCTION .. 5 Hydrology Definition .. 5 Exceedance Probability .. 5 Q Definition .. 5 Estimating Flow Rate .. 6 HYDROLOGIC-ANALYSIS PROCEDURES .. 6 Data Gathering and Preliminary Studies .. 6 Hydrologic Factors .. 6 Determining drainage Area .. 7 Determination of Soil, Vegetation, and Land-Use Factors .. 8 Determination of Time of Concentration .. 8 (01) Sheet-Flow Method .. 9 (02) Shallow-Concentrated-Flow Method .. 9 (03) Open-Channel-Flow Method .. 10 (04) Lag Time .. 10 (05) Storage and Flood-Control Facility.

CHAPTER 202 HYDROLOGY 202-1.0 INTRODUCTION Highway drainage structures must be sized to convey a specific flow rate, Q, which satisfies some established standard. Determining structure size and type is generally one of the earliest activities undertaken in the design process. Various hydrologic methods can be used to estimate Q. Multiple ...

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Transcription of CHAPTER 202

1 INDIANA DEPARTMENT OF TRANSPORTATION 2013 DESIGN MANUAL CHAPTER 202 Hydrology Design Memorandum Revision Date Sections Affected 14-02 Feb. 2014 , Figure 202-3B, Figure 202-3C Page 2 2013 Indiana Design Manual, Ch. 202 TABLE OF CONTENTS LIST OF FIGURES .. 4 INTRODUCTION .. 5 Hydrology Definition .. 5 Exceedance Probability .. 5 Q Definition .. 5 Estimating Flow Rate .. 6 HYDROLOGIC-ANALYSIS PROCEDURES .. 6 Data Gathering and Preliminary Studies .. 6 Hydrologic Factors .. 6 Determining drainage Area .. 7 Determination of Soil, Vegetation, and Land-Use Factors .. 8 Determination of Time of Concentration .. 8 (01) Sheet-Flow Method .. 9 (02) Shallow-Concentrated-Flow Method .. 9 (03) Open-Channel-Flow Method .. 10 (04) Lag Time .. 10 (05) Storage and Flood-Control Facility.

2 10 (06) Storm-Drain-Flow Method .. 10 (07) Stormwater-Storage-Flow Method .. 12 Determination of Precipitation Rate .. 12 Determining Discharge .. 12 ACCEPTED METHODOLOGIES .. 13 Definition .. 13 Selection of Discharge-Computation Method .. 13 Q500 Determination for Scour Calculations [Added Feb. 2014] .. 14 Selection of Discharge for Pump Station .. 15 Selection of Methods Table .. 15 DOCUMENTATION FOR HYDROLOGY .. 15 Culvert or Bridge .. 16 (01) Narrative .. 16 (02) drainage Area .. 16 (03) Rational or Unit Hydrograph Method .. 16 (04) Other Methods .. 17 Pavement or Storm drainage .. 17 (01) Narrative .. 17 (02) drainage Area .. 17 2013 Indiana Design Manual, Ch. 202 Page 3 (03) Hydrologic Computations .. 17 Stormwater Storage .. 18 (01) Narrative.

3 18 (02) drainage Area .. 18 (03) Hydrologic Computations .. 18 REFERENCES .. 18 FIGURES .. 20 Page 4 2013 Indiana Design Manual, Ch. 202 LIST OF FIGURES Figure Title 202-2A Example of TR-55 Time of Concentration Worksheet 202-2B Manning s Roughness Coefficient, n, for Sheet Flow 202-2C Manning s Roughness Coefficient, n, for Channel Flow 202-2D Average Velocities for Estimating Travel Time for Shallow Concentrated Flow 202-2E Rational-Method Runoff Coefficient, C 202-2F Curve-Number Table 202-3A Selection of Discharge-Computation Method 202-3B Example of Graphical Method for Determining the EP [Added Feb. 2014] 202-3C Regional Multipliers for Determination of Annual EP [Added Feb. 2014] 2013 Indiana Design Manual, Ch. 202 Page 5 CHAPTER 202 HYDROLOGY INTRODUCTION Highway drainage structures must be sized to convey a specific flow rate, Q, which satisfies some established standard.

4 Determining structure size and type is generally one of the earliest activities undertaken in the design process. Various hydrologic methods can be used to estimate Q. Multiple methods of obtaining estimates can sometimes be used, and the engineer determines the best Q to use at that site. A specific method of determining Q can sometimes be prescribed. Hydrology Definition Hydrology is a broad science encompassing many disciplines relating to water. In highway engineering, hydrologic studies are primarily concerned with surface-water runoff and determining design Q. A hydrologic analysis is a prerequisite to identifying flood-hazard areas and determining the locations at which construction and maintenance will be unusually expensive or hazardous.

5 Exceedance Probability Exceedance probability is a statistical term that states the probability of a specific value or event being equaled or exceeded. Established rainfall events for design purposes are described, such as 100-year storm, Q100, 1% storm, return period, exceedance probability, etc. The INDOT preferred method of stating a standard rainfall event is as exceedance probability, EP. An established standard can be set as INDOT policy or it may be set by an outside authority such as a city, county surveyor for a legal drain, IDNR, State law, or other jurisdictional agency. Therefore, although the standard may not be originally stated in INDOT s preferred terms, it should be restated as EP. Q Definition Q is the common denotation of the total flow rate.

6 It is dimensioned in cubic feet per second for INDOT design. Depending on the project, there can be different Qs of interest. For example, for sizing a bridge based on backwater criteria, the design Q is that for a 1% EP. However, for scour Page 6 2013 Indiana Design Manual, Ch. 202 analysis on a new bridge, the 1% EP and the EP both should be evaluated. A storm drain, channel, ditch, local driveway, etc. each have a specific EP defined for design purposes. Estimating Flow Rate The importance of accurate estimates of Q cannot be overstated. An error committed in this part of the design can directly lead to an incorrectly-sized drainage structure. An oversized drainage structure implies that money can be wasted. An undersized structure increases the risk of potential flooding and maintenance issues.

7 Existing-structure size should be considered in designing a replacement structure. Long term legal, financial, or operational impacts can be expected if undersized structures are consistently installed. Most highway- drainage structures are constructed using taxpayer-supported funding. As such, an INDOT project should present the correct stewardship. The designer should utilize sound hydrologic methodology in the development of the design. HYDROLOGIC-ANALYSIS PROCEDURES The following is a discussion of policies and procedures which apply to hydrologic analysis. For more information, see the AASHTO Highway drainage Guidelines. Data Gathering and Preliminary Studies The data to be gathered for a hydrologic study includes, but is not limited to, topographic maps, soil maps, aerial photographs, stream-flow records, historical high-water elevations, flood discharges, rainfall curves, or locations of hydraulic features such as reservoirs, water projects, or designated or regulatory floodplains, and contemporaneous interviews with affected property owners.

8 Hydrologic Factors Hydrologic factors are site specific and should be evaluated by an engineer proficient in the practice of hydraulic and hydrologic engineering to understand their effect on the runoff from a watershed. Many factors may be subject to alteration over time and whether that potential needs to be addressed should also be evaluated. Some of these factors include the following. 2013 Indiana Design Manual, Ch. 202 Page 7 1. drainage -Basin Characteristics. These include size, shape, slope, land use, geology, soil type, surface infiltration and storage, and the maturity of the watershed s drainage system. 2. Stream-Channel Characteristics. These include geometry and configuration, natural and artificial controls, channel modifications, aggradations or degradations, ice, debris, and routine maintenance.

9 3. Floodplain Characteristics. A floodplain is defined as a strip of relatively smooth land bordering a stream and is flooded at a time of high water. Due to its continually changing nature, a floodplain or other flood-prone area should to be examined relative to how it can affect or be affected by development. Flooding is a natural and recurring event for a river or stream. 4. Meteorological Characteristics. These include observable weather events which are explained through the science of meteorology. Those events are bound by the variables that exist in the atmosphere: They include temperature, air pressure, water vapor, and the gradients and interactions of each variable, and how they change in time. INDOT s concerns are for intensity and duration of specific storm events.

10 The site-specific data used for an INDOT hydrologic study appears in NOAA Atlas 14. Determining drainage Area With the exception of a very large watershed, a drainage -area boundary should be delineated on a topographic map. The most readily-available topographic maps are USGS Quadrangle maps. Many counties have had more-detailed topographic surveying done that is mapped to 1- or 2-ft contours. The availability of topographic-survey mapping can be determined from the Indiana Geographic Information Council website or by contacting the county surveyor. Initial area determination can be obtained using computer-based automatic delineation programs. Web-based and easily-accessed programs include USGS STREAMSTATS and HYMAPS-OWL. STREAMSTATS can be accessed at the USGS website.


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